One out of ten Bostonians has been exposed to coronavirus, according to initial testing of people in four Zip codes

The city and Massachusetts General Hospital today released results from the recent random Covid-19 testing in East Boston, Roslindale and part of Dorchester: 9.9% of the people tested positive for Covid-19 antibodies - and that roughly 1 in 40 people with no symptoms were currently infected with the virus and so potentially infectious themselves.

Hospital and city public-health staffers tested 750 people in the neighborhoods for both antibodies, through a finger-prick blood test, and for active infection, through a nasal swab. A positive result on the first test means the person was exposed to enough of the virus to develop antibodies to it, but is likely no longer infectious; a positive result on the second test indicates the person's body is currently battling an infection. People who had previously tested positive for infection or who were showing active symptoms were excluded from the tests.

In announcing the results, Mayor Walsh said:

We can draw two preliminary conclusions from the results of this study. First, that the actions we took early on in this pandemic made a real difference in slowing the spread and, second, that the majority of our population still have not been exposed to the virus. This underscores what we already know, that we have to move cautiously and stay focused on what got us this far. This can be done by a gradual, phased-in approach to reopening that includes clear health criteria and safety guidelines for each industry and depends on testing and hospital metrics reaching certain benchmarks, and continuing to move in the right direction.

The median age of people getting a test was 42.4; 61.6% were female, the rest male. In terms of neighborhoods: 36.8% are from Roslindale, 25.1% are from East Boston, 23.2% are from 02125 in Dorchester and 14.9% are from 02121 in Dorchester. Also: 62% are white, 18.7% are Black/African-American, 12% are Latinx/Hispanic, 2.3% are Asian/Pacific Islander and .13% are American Indian/Alaska Native. 1.6% preferred not to say and 1.6% are unknown. There were no significant differences in COVID-19 or antibody rates by race or ethnicity in this sample, the city says.

Mass General President Peter Slavin added:

The testing that the teams from Boston and the MGH conducted shows that approximately 90 percent of the city's residents have not yet been exposed to the virus. We also know that COVID-19 will be with us for a while. It is vital therefore that we be thoughtful and careful about reopening, and that we continue to take actions - wearing masks, physical distancing, working from home when possible, limiting gatherings - that can prevent another surge of the disease.

People are hopeful over things that are way less likely than one in ten. Things with a 10% chance of happening happen pretty often, in the grand scheme of things. A 10% chance means that you probably haven't already gotten it, but if it turns out you already have you shouldn't be too surprised. So... continue to be careful, but keep in mind that it's extremely possible you're one of the lucky ones.

Wouldn't the ideal result be that we have more than 50% antibodies which would mean herd immunity might be nearer than hoped? If 90% of us haven't been exposed to this, then social distancing, etc... will have to continue for a long time, no? To be clear, I'm not advocating that we should pull a Sweden here to get everyone exposed but it would have been great if covid-19 had been more widespread and less bad than thought prior to the lockdown, which is what a big number of anti-bodies would have indicated.

I think the plan by medical experts and government is to slowly open up, and as we do this our antibodies % will go up as well over time (8-12 months?) I am sympathetic to the economic impact of this though. I think with masks and our social distancing practices we can do well by opening up sooner than later. I am also not an economist, but I'm not sure this country can hold out much longer economically like this without drastic consequences.

borrowing rates are nearly zero. The feds could add $1T to the deficit with out any real impact on anyone and keep a lot of people out to poverty and businesses from failing. It's the politics that are the problem, not the economics.

We see many false negatives (tests where no antibody is detected despite the fact we know it is there) and we also see false positives. None of the tests we have validated would meet the criteria for a good test. This is not a good result for test suppliers or for us.” -- Sir John Bell, Professor of Medicine, Oxford University

It seems that citizens seeking a test may avoid the cost and hassle by simply putting all of the possibilities on sticky notes, crumpling those and then pulling one from a hat. The chances of pulling a note indicating positive, negative, false negative, false positive etc. are probably about as close to the "accuracy" of an approved test. It's important to recall that the goal of the lockdown was to prevent hospitals from being overwhelmed by the doomsday predictions which, in retrospect, proved grossly exaggerated. Nobody was denied a ventilator or hospital bed and now there is a surplus of both. We know that hospitals are partially vacant with many trained, healthcare professionals on the unemployment line, not the front line. Life is full of risks and there is actual proof that the risk of death here is miniscule for those healthy under age 60. Let's remember the lives lost and protect those still in the vulnerable categories. The ambitious goal of the lockdown has been met and far exceeded, so let's stop the insanity.

This study oversampled women over men, and the Roslindale population by a huge amount. That doesn’t mean it’s useless, but the results need to be weighted for these sampling imbalances.

The population of the total area sampled is about 150,000. Roslindale makes up 20% of this population, but 37% of those sampled were from 02131. The Dorchester zip codes have about twice the population of Roslindale, but the number of people sampled is only 1% larger.

It’s still valuable information and it proves that so far there is not a huge number of asymptomatic people who have acquired immunity under the radar. But what we really need is a scientific selection of random test-takers from all over the city, and hopefully all over the state.

I was hoping that the antibody results would be high, like they were in Chelsea, but this is still good info. I can't wait until we're doing 50,000+ tests per day like Governor Baker suggests we will. With that data people will be much better able to judge the risks.

I'm glad to see that they did some testing in my area (02125), though I actually wish I had known and had been able to volunteer, had that been possible. I'm "healthy" - meaning I'm either not infected at all or I'm just asymptomatic, but I yearn for the time when we can all get tested even without suspicion of symptoms, etc. I know that infections can happen anytime, but I think a lot of us would find some needed peace of mind knowing where we are with things after the last few months. Any kind of reassurance would be helpful for all of us right now - even, I would think, getting a positive test result (I'd rather know than not know).

Hoping that sooner, rather than later, testing can be readily available as a matter of course. Meanwhile, it's great to know that random testing like this IS being done, and I hope it continues.

Was this random testing? Or were people "randomly" chosen from a pool of City of Boston employees? Because the people I know who were tested are City employees and that's how they seemed to understand it.

They would have had to select addresses at random and survey at least one occupant at each address. I read an article what seems like an eternity ago that suggested that doing that would be a better way of determining the spread of the virus.